Highlights from Recent Cancer Literature
Journal: Cancer Research (2013)

Soroceanu L, Murase R, Limbad C, Singer EL, Allison J, Adrados I. Id-1 is a key transcriptional regulator of glioblastoma aggressiveness and a novel therapeutic target. Published OnlineFirst December 13, 2012; doi: 10.1158/0008-5472.CAN-12-1943.The key molecular pathways leading to glioblastoma multiforme (GBM) initiation and progression are poorly understood, thereby representing a challenge to GBM management. In this study, Soroceanu and colleagues characterized the critical role of Id-1 (inhibitor of DNA binding 1, dominant negative helix-loop-helix protein, ID1) in GBM initiation and progression. The expression level of Id-1 correlated well with GBM invasiveness in multiple cell lines. Remarkably, a significant number (70%) of primary GBM-derived cells expressed high levels of Id-1 when grown as neurospheres in defined media, a method that enriches for cancer stem and progenitor cells. The increased expression levels of Id-1 also correlated well with higher grades of primary GBM tumors. Knockdown of Id-1 in U251 GBM cells not only reduced invasion but also altered mesenchymal phenotype, as indicated by the downregulation of several mesenchymal markers, including vimentin, α-tubulin, Snail, and MT1-matrix metalloprotease, and produced a profound change in cell morphology. Notably, Id-1 was shown to regulate the growth of primary GBM and U251 cell-derived neurospheres by specifically interacting with SOX2, a key GBM initiation factor. Inhibition of Id-1 expression led to alteration of multiple genes associated with GBM progression, which, in turn, is associated with increased therapeutic resistance, and significantly enhanced survival of mice in an orthotopic setting, in which ID-1 U251 cells depleted for ID-1 were injected intracranially. Additionally, treatment of Id-1 overexpressing U251 or primary GBM cells with the ID-1 inhibitor cannabidol, reduced invasiveness and neurosphere formation by downregulating Id-1 expression and modulating the activity of ERK, AKT and SOX2. Moreover, Cannabidiol treatment inhibited GBM dispersal ex vivo and growth in vivo of U251 cells overexpressing Id-1. In these contexts, Id-1 appears to be a key regulator of GBM initiation and progression and an attractive molecule for biomarker and therapeutic development.Ruark E, Snape K, Humburg P, Loveday C, Bajrami I, Brough R, et al. Mosaic PPM1D mutations are associated with predisposition to breast and ovarian cancer. Nature 2012 Dec 16. [Epub ahead of print].Rare genetic mutations have been shown to increase the risk for development of cancer. For example, very rare loss-of-function mutations in genes associated with DNA damage response confer high predisposition to breast and ovarian cancer. However, technical and analytical limitations have historically constrained research to unravel the role of rare genetic variants in tumorigenesis. In this study, Ruark and colleagues used new and highly sensitive deep sequencing technologies to show that point-truncating variant (PTV) nonsense mutations in the PPM1D gene, a negative regulator of p53 tumor suppressor activity, are associated with increased cancer susceptibility. The authors carried out a focused deep sequencing strategy to sequence over 500 genes associated with DNA repair in blood samples from 1,150 patients with breast and/or ovarian cancer. They uncovered over 1,000 different point-truncating mutations and determined that PPM1D was most strongly associated with PTVs in their panel. Case-control Sanger sequencing of 13,642 individuals uncovered 25 PPM1D PTVs in breast/ovarian cancer patients versus only 1 PPM1D PTV in the control population, strongly suggesting that PPM1D PTVs may predict predisposition to breast and ovarian cancers. Interestingly, Sanger sequencing chromatograms, deep PCR amplicon sequencing, and multiplex ligation-dependent probe amplification experiments suggest that the PPM1D point truncation mutation was mosaic in lymphocyte DNA. Point-truncating variants introduce premature stop codons into translated mRNAs and therefore are normally associated with downregulation of protein activity. However, Ruark and colleagues show that PTV mutations cluster within the last exon of the PPM1D gene, which may allow the shortened mRNA to evade nonsense-mediated decay. Additionally, the PPM1D protein is truncated downstream of the phosphatase catalytic domain, which could allow it to retain its p53 suppressive function. To elucidate the effect of PPM1D PTV mutants on p53 activation, the authors transfected U2OS human osteosarcoma cells with PPM1D cDNA expression constructs harboring 2 different PTV mutant alleles. They found that, upon DNA damage by ionizing radiation, p53 activity was suppressed in cells transfected with mutant PPM1D compared with cells transfected with wild-type PPM1D. Therefore, PPM1D PTVs may encode hyperactive PPM1D isoforms, conferring a gain-of-function effect. Most interestingly, the authors show that the PPM1D PTVs are not present in the tumors but rather are found in blood and, to a much lesser extent, stroma. These data suggest the possibility of a novel mechanism by which PPM1D point-truncating mutations in DNA repair genes may predispose patients to cancer susceptibility without a direct function in the cancer cells. However, much more research must be carried out to determine whether and how PPM1D PTVs contribute to cancer development. The authors' findings represent a possible new research direction into the role of rare PTVs in tumorigenesis as well as new early detection biomarkers that may identify genetic predisposition to cancer.Britschgi A, Andraos R, Brinkhaus H, Klebba I, Romanet V, Müller U, et al. JAK2/STAT5 inhibition circumvents resistance to PI3K/mTOR blockade: a rationale for cotargeting these pathways in metastatic breast cancer. Cancer Cell 2012;22:796–811.The phosphoinositide 3-kinase (PI3K)/mTOR pathway is frequently hyperactivated in human cancers and contributes to many of the hallmarks of cancer. Twenty-six different drugs that target this pathway are currently in clinical trials. However, targeted therapies used as single agents can have impressive but short-lived antitumor effects because they disrupt a myriad of negative feedback loops, resulting in activation of other aberrant signaling pathways in the malignant cells. If these feedback loops can be identified and targeted in combination or in sequence, then more sustained effects may be achieved. Using cell line and mouse models of the triple-negative subtype of human breast cancer, Britschgi and colleagues identified an important positive feedback loop that is generated by drugs that target the PI3K/mTOR pathway. They found that PI3K/mTOR inhibitors increased IRS1-dependent activation of JAK2/STAT5 and secretion of the tumor-promoting cytokine interleukin (IL)-8. The activation of JAK2/STAT5 was biphasic: AKT was reactivated, increasing malignant cell survival, and in mouse models, systemic levels of the tumor-promoting cytokine IL-8 were increased. This was particularly important as a subpopulation of highly tumorigenic and metastatic breast cancer cells expressed the IL-8 receptor CXCR1. Genetic or pharmacologic inhibition of JAK2 abrogated the feedback loop in the breast cancer cells. When Britschgi and colleagues tested a combination of PI3K/mTOR and JAK2 inhibitors in human breast cancer xenografts from cell lines and from primary tumors, they found a synergistic reduction in tumor growth, decreased tumor seeding, and metastasis. When treatment was started after primary tumors were excised, the combination therapy increased overall mouse survival. Triple-negative breast cancer is an aggressive cancer with limited treatment options. These experiments provide a rationale for combining PI3K and JAK2 inhibitors in patients with this malignancy and in other cancers in which PI3K/mTOR is subverted.Sebastian C, Zwaans BM, Silberman DM, Gymrek M, Goren A, Zhong L, et al. The histone deacetylase SIRT6 is a tumor suppressor that controls cancer metabolism. Cell 2012;151:1185–99.Compared with nonneoplastic cells, cancer cells adjust their energy metabolism, an adjustment thought to be essential to tumor growth. A major change in metabolism in cancer results from altered glucose metabolism. Under normal (nonneoplastic) aerobic conditions, glucose is metabolized by glycolysis to pyruvate. Pyruvate in turn goes to mitochondria to enter the Krebs cycle for oxygen-dependent ATP generation. Under anaerobic conditions, mitochondrial respiration does not occur and the pyruvate is instead converted to lactate. Cancer cells rely on a glycolysis-only metabolism of glucose, even in the presence of oxygen, a phenomenon known as aerobic glycolysis or the Warburg effect. A major issue in cancer metabolism is whether the Warburg effect is a secondary response to cell proliferation and survival signals that are promoted by oncogene activation/tumor suppressor gene inactivation or instead whether aerobic glycolysis is indeed a driving force for tumorigenesis. To address this, the authors examine SIRT6, a member of the sirtuin family of NAD+-dependent protein deacetylases involved in metabolic homeostasis. They first used embryo fibroblasts (MEF) from Sirt6 knockout mice to show that loss of SIRT6 promoted tumorigenesis. Second, they showed that Sirt6-knockout MEFs exhibited enhanced aerobic glycolysis, with increased glucose uptake and lactate production. In fact, Sirt6-knockout MEFs showed evidence of glucose addiction and were more prone to cell death upon glucose withdrawal. These results indicated that SIRT6 functions as a tumor suppressor by inhibiting the switch to aerobic glycolysis. To confirm this observation, they blocked glycolysis via PDK1 knockdown, which inhibited the conversion of pyruvate to lactate. Knockdown of PDK1 resulted in reduced anchorage-independent growth and tumorigenesis, suggesting that aerobic glycolysis was critical for tumor-suppressor functions of SIRT6. They elucidated the mechanism by showing that SIRT6 loss resulted in activation of a subset of MYC-dependent genes. These MYC-dependent genes in turn promoted both glutamine metabolism and biogenesis of ribosomes. Interestingly, loss of SIRT6 did not increase expression of MYC or alter protein stability. Rather, loss of SIRT6 promoted an epigenetic change (H3K56 acetylation on the promoter region of ribosomal protein genes), a histone mark involved in transcriptional regulation. Further, the authors showed that MYC function was, in fact, required for the tumorigenic phenotype that resulted from loss of SIRT6, as MYC knockdown in SIRT6-deficient MEFs reduced tumorigenesis. To establish the relevance of these findings to human tumors, the authors examined DNA copy data from The Cancer Genome Atlas to show that SIRT6 was lost at the DNA level in up to 20% of human tumors. These data were supported by immunohistochemical studies in pancreatic carcinoma, showing decreased protein expression. Additionally, loss of protein expression was correlated with shorter relapse time in node-positive patients with colorectal adenocarcinoma. Finally, the authors used a conditional (floxed) Sirt6 allele to show that loss of Sirt6 promoted an increase in colorectal adenomas in a mouse model of colorectal polyposis. Taken together, these results show that SIRT6 functions as a tumor suppressor via repression of ribosomal gene expression and suggest aerobic glycolysis as a key driver of the tumorigenic process.Note: Breaking Advances are written by Cancer Research Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.